Method for fabricating burning rate strands and tensile specimens



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Sept. 12, 1967 3,341,636 I: ATE, STRANDS METHOD FOR FABRICATIN AND TENSILE SP 5 Sheets-Sheet 1 Filed Jan. 17, 1966 FLOYD L. HILL INVENTOR ATTORNEY F. L. HILL Sept. 12, 1967 METHOD FOR FABRI AND T CATING BURNING RATE STRANDS ENSILE SPECIMENS Flled Jan. 17, 1966 5 Sheets-Sheet 2 FLOYD L. HILL INVENTQR ATTORNEY Sept. 12, 1967 F L H|LL 3,341,636

METHOD FOR FABRICKTING BURNING RATE STRANDS AND TENSILE SPECIMENS Filed Jan. 17, 1966 5 $heets5heet 3 FLOYD L. HILL INVENTOR ATTORNEY 3,341,636 METHGD FOR FABRICATING BURNING RATE STRANDS AND TENSILE SPECIMENS Floyd L. Hill, Huntsville, Ala., assignor to Thiokol Chemical Corporation, Bristol, Pa., a corporation of Delaware Filed Jan. 17, 1966, Ser. No. 521,150 2 Claims. (Cl. 2643) ABSTRACT OF THE DISCLOSURE A method for fabricating self-inhibiting burning rate strands of solid propellant comprising the following steps, lowering a mold having cavities therein into uncured liquid propellant, applying a vacuum source to the mold to draw the uncured liquid propellant into the mold cavities, continuing the application of the vacuum source until all air bubbles have been drawn through the mold cavities, curing the uncured liquid propellant and then applying air under pressure to the mold cavities to eject the cured solid propellant from the mold cavities to provide the strands of solid propellant that are to be used as test specimens.

This invention relates to improvements in the method for fabricating strain-free, constant diametric, cylindrical solid propellant strands each of which has a single, continuous outer surface and are to be used as a self-inhibiting burning-rate strand or for high-integrity bonded-end tensile specimens.

One of the most important steps in fabricating solid propellant strands that are to be used for the test specimens in the fabrication of such strands in a simple and economical manner. The need for obviating any waste in the fabrication of such strands was greatly magnified when costly high-energy solid propellant development was ing-rate strands of diminutive size, uniform cross-section, and high reliability. It was also necessary that the tensile specimens possess high-integrity, be void-free and strainfree, as well as, also have high reliability. Therefore, the strands fabricated by the instant invention had to achieve low fabricating costs and possess minimal material usage and waste in their fabrication.

The conventional split molds in use at the present time have to be coated with Teflon or some other conventional release agent and the use of such materials often rendered the strands unsuitable for test purposes. The strands were often filled with air pockets, had ragged outer surfaces and such conditions resulted in highly strained and damaged strands. The strands fabricated under the techniques of the instant invention were found to provide better burning-rate data and when using the strands as'microtensile specimens, great savings were obtained when they were fabricated from costly high-energy solid propellant.

Other conventional methods in producing the solid propellant strands also included sawing, guillotine slabbing and die-cutting, however, the strands fabricated by such known techniques possessed sharp corner edges and ragged outer surfaces that induced flashing in burningrate strands and considerable waste and expense in inhibiting operations.

With tensile specimens it is also highly desirable for such specimens to possess a uniform strain field and the cylindrical configuration of the strands fabricated by the the conventional square, figurations that are in use at the present time.

The fact that the strand has a known uniform crosssection also obviates the need for measuring each strand nited States Patent fire 3,341,636 Patented Sept. 12, 1967 to be tested thus, considerable time is saved and all measurement errors that might occur during the measuring of the strand are eliminated. Thus, with a known and constant diameter a cylindrical solid propellant strand affords the most uniform flame field during flame propagation along the length of the strand and the most uniform strain field during the course of a tensile test. Also the use of an inhibitor in a strand skin which is infinitesimally tinuous with no exposed oxidizer particles that would subject the strand to flashing or leaching during storage periods.

It is an object of this invention therefore, to provide a solid propellant strand that is of cylindrical configuration, the most desirable configuration that is available for de termining mechanical property characteristics.

It is another object of this invention to provide a method for fabricating a solid propellant strand so that durstorage periods for the It is a further time are entirely eliminated.

It is a still further object of this invention to provide a method for fabricating a solid propellant strand that bepossessing higher integrity.

With the above and other objects and advantages in view, the invention consists of the novel details as will appear from a reading when combined with which:

FIGURE 1 is a sectional view, partly in elevation and partly broken away, to illustrate the apparatus of the instant invention;

FIGURE 2 is a of FIGURE 1;

FIGURE 3 is a vertical sectional the line 2-2 of FIGURE 2; and

FIGURE 4 is a sectional view of the mold of FIGURE 2 illustrating the manner in which the strands are removed from the mold.

Referring more in detail to the drawings, wherein like parts are designated by like reference numerals, the refplan view of the mold of the apparatus view of the mold on bodying the invention.

The apparatus 10 comprises a reservoir 11 into which is poured a supply of uncured solid propellant 12.

positioned FIGURE 1.

The reservoir 11 can be of any shape or size depending upon the supply of uncured solid propellant 12 that is required to fabricate the strands 14. The material used for producing the reservoir 11 can be one of many materials that are available for its production and may be light or heavy, depending on the material to be used and the gauge thereof.

The mold 13, as shown in FIGURES 2 and 3, is of cylindrical formation and provided with a plurality of equally spaced circular rows 16 of equally and uniformly spaced molding cavities 17 that extend completely through the breadth of the mold 13.

The mold 13 is fabricated from an elastomeric material and the following formulation has proved successful in its fabrication:

Casting a mixture of 1% pounds of GE. RTV-ll Silicone Rubber and 0.5 gram of Nuocure 28 curing agent into a cylindrical form in which are positioned in equally and uniformly spaced relation to each other a plurality of precision diameter metal rods which produce the circular rows 16 of molding cavities 17 and then curing the mixture, under vacuum, for 40 minutes at a temperature of 135 degrees Fahrenheit. The cured mold 13 was readily extractable from the cylindrical form, because of the non-bonding nature of the Silicone Rubber and the molding cavities 17 even formed with void free circumferential inner wall surfaces.

The hood or funnel 15 comprises a circular wall portion 18 which completely surrounds the outersurface of the mold 13 and the tolerance between the outersurface of the mold 13 and the inner surface of the wall portion 18 is as close as possible to permit the hood or funnel 15 to be easily slipped over the mold 13 and yet not permit excess leakage of air between the contacting surfaces of both.

A dome portion 19 of substantially frusto-conical formation has the lower peripheral edge thereof integral with the upper peripheral edge of the wall portion 18 and a hose coupling 20 at the apex 21 thereof. A hose 22 from a conventional vacuum source, not shown, is connected by the hose coupling 20 to the hood or funnel 15. The vacuum source will, therefore, When in operation, create a vacuum environment within the chamber formed by the hood or funnel 15 which extends from the upper surface of the mold 13 to the apex 21 of the hood or funnel 15.

The members of the apparatus previously described are adapted to function in the following manner to fabricate the strands 14.

The hood or funnel 15 is slipped over the mold 13 and the mold 13 is manually lowered by means of the hood or funnel 15 into the uncured solid propellant 12. Any type of suitable support may then be employed to sustain the mold 13 in the position shown in FIGURE 1. The vacuum environment is then created within the hood or funnel 15 by means of the hose 22 that is connected to the vacuum source and the uncured solid propellant 12 is drawn into the mold cavities 17, shown in FIGURE 1. Sufficient uncured solid propellant 12 is drawn through the mold cavities 17 to be certain that all air bubbles are concentrated in the portions of the strands 14 that extend above the upper surface of the mold 13. Such portions 23 with the air bubbles formed therein, as shown in FIGURE 1, are then cut off of the strands 14 after the hood or funnel 15 has been removed and are returned to the reservoir 11.

After the removal of the mold 13 from immersion in the uncured solid propellant 12 and the removal of the hood or funnel 15 from the mold 13 and after the portions 23 have been removed from the strands 14, the mold 13 is placed in a curing area for the curing of the strands 14.

After the mold 13 has been removed from the curing .4 area the mold 13 is positioned on a suitable support, as shown in FIGURE 4, for the removal of the strands 14 from the molding cavities 17 in the mold 13. An air hose fitting 24 that is connected to an air source that will provide a low-pressure air source of approximately 5.0 to 10.0 p.s.i. will be applied to each of the mold cavities 17 at one end thereof and the force of the low pressure air will aid in extracting the strands 14 from the mold cavities 17.

As shown in FIGURE 4 at 25, the low pressure air slightly expands each of the mold cavities 17 to which it is individually applied by means of the nozzle 26 of the air hose fitting 24 because of the inherent flexible nature of the Silicone Rubber from which the mold is fabricated and such action permits the individual strand 14 to be gently and easily removed from its associated mold cavity 17, all as shown in FIGURE 4.

After all of the strands 14 have been removed from the mold cavities 17 in the mold 13, the strands 14 are ready to be used for test purposes and the mold 13 is ready to be reused as previously described.

There has therefore been provided a method for fabricating strands as contemplated by the instant invention and it is believed that from the foregoing description an understanding of the invention will be clear to those skilled in the art, it being understood that any variations from the procedure and apparatus, as previously set forth, may be adhered to providing such variations fall within the spirit of the invention and the scope of the appended claims.

Having thus described the invention what is claimed as new and desired to be secured by Letters Patent is:

1. A method for fabricating strands of solid propellant comprising the steps of lowering a mold having mold cavities therein into a reservoir of uncured liquid propellant, applying a vacuum source to the mold to draw the uncured liquid propellant into the mold cavities, continuing applying the vacuum source until all of the uncured liquid propellant containing air 'bubbles has been drawn through the mold cavities, curing the uncured liquid propellant and then applying air under pressure to the mold cavities to eject the cured solid propellant from the mold cavities.

2. A method for fabricating strands of solid propellant comprising the steps of lowering a flexible mold having mold cavities therein into a supply of uncured liquid propellant, applying a vacuum source to the mold to draw the uncured liquid propellant into the mold cavities, continuing applying the vacuum source until all of the uncured liquid propellant containing air bubbles has been drawn through the mold cavities, cutting the excess propeliant away from said mold, curing the uncured liquid propellant in the mold cavities and then applying air under pressure to the mold cavities to eject the cured solid propellant from the mold cavities.

References Cited UNITED STATES PATENTS L. DEWAYNE RUTLEDGE, Primary Examiner, 

1. A METHOD FOR FABRICATING STRANDS OF SOLID PROPELLANT COMPRISING THE STEPS OF LOWERING A MOLD HAVING MOLD CAVITIES THEREIN INTO A RESERVOIR OF UNCURED LIQUID PROPELLANT, APPLYING A VACUUM SOURCE TO THE MOLD TO DRAW THE UNCURED LIQUID PROPELLANT INTO THE MOLD CAVITIES, CONTINUING APPLYING THE VACUUM SOURCE UNTIL ALL OF THE UNCURED LIQUID PROPELLANT CONTAINING AIR BUBBLES HAS BEEN DRAWN THROUGH THE MOLD CAVITIES, CURING THE UNCURED LIQUID PROPELLANT AND THEN APPLYING AIR UNDER PRESSURE TO THE MOLD CAVITY TO EJECT THE CURED SOLID PROPELLANT FROM THE MOLD CAVITIES. 